Ring saws have a circular disc-shaped saw body with a circumferential edge. A cutting ring is mounted for rotation about the circumferential edge. The cutting ring has all inner annular surface with gear teeth. At least one drive gear is rotatably mounted in the saw body. The drive gear engages the gear teeth on the inner annular surface to rotate the cutting ring.
Ring saws used in the forestry industry for cutting down trees are subjected to significant radial and axial loading. Radial loading occurs as the cutting ring engages the tree. Axial loading occurs as cutting progresses and the weight of the tree bears upon the cutting ring.
Canadian Patent 1,291,396 discloses an improved ring saw which addresses the "difficulty in mounting the ring saw on the saw body to take stresses such as those applied by a tree resting on the saw". This ring saw has a track in which the cutting ring rotates. The track has a number of arcuate "bearing grooves" arranged in end to end relation in which are received "bearing strips". The bearing strips are described as being "made from a suitable plastic material having a low coefficient of friction". The cutting ring has an inwardly projecting annular flange. Three idler gears cooperate with the inwardly projecting flange to help center the cutting ring during rotation and cutting operations. The inwardly projecting flange is sandwiched between the bearing strips to withstand axial loading.
There are a number of problems with the teachings of Canadian Patent 1,291,396. Firstly, the engagement between the bearing strips and the inwardly projecting annular flange on the cutting ring is intended to serve a dual purpose of a bearing surface and a sealing surface. However, the plastic bearing material of the bearing strips will invariably experience deterioration under heavy axial loading which reduces its ability to function as a sealing surface. Without effective sealing dirt and debris enter into the gears and decrease the operational life of the gears. Secondly, the number of rotations per minute of the cutting ring is limited by the friction properties of the plastic bearing material of the bearing strips. Thirdly, when the gears perform a centering function they are subjected to radial loads, which reduces the operational life of the gears.